A long-standing challenge in palladiumn .pi.-allyl chemistry has been the use of oxygen nucleophiles to generate O-allylated species with good regio- and enantioselectivity (FIG. 1A). Although phenols and carboxylates have been shown to be good nucleophilic partners for this type of reaction, alcohols have generally given poor results due to their sluggish reactivity and moderate regioselectivity.
Only a few literature examples employing alcohols as nucleophiles with Pd .pi.-allyls have appeared, most of which involve simple O-allylation of alcohols (Lakhmiri et al., 1989), in which neither regio- nor stereoselectivity is an issue (Guibe et al, Lakhmiri et al., 1990a, 1990b). Several examples of Pd(0)-catalyzed O-glycosylation of sugars to produce 1,4-disaccharides have been reported by Sinou et al.
Trost et al have investigated a number of strategies for the delivery of oxygen nucleophiles (or hydroxyl equivalents) to Pd .pi.-allyl complexes arising from vinyl epoxides. Specifically, Trost and Tenaglia (1988) examined the use of alkoxystannanes for the regioselective 1,2-addition of alkoxy groups to vinyl epoxides. The reaction was not stereoselective, and its scope of this reaction was somewhat limited in that good yields were obtained only by the use of cyclic stannylene diethers.
As part of a total synthesis of zoapatanol, Trost and Ito (1993) developed the use of triphenylsilanol as a water surrogate that selectively adds in a 1,4sense to vinyl epoxides. The products of this reaction were formed mainly with the E-geometry. However, attempts to repeat this work using a chiral ligand (Bunt, 1996) were unsuccessful.
A general method for regio- and enantioselective addition of alcohols to vinyl epoxides, therefore, has not yet been reported. Such a procedure would be extremely valuable in the preparation of chiral precursor molecules to biologically important molecules, or as building blocks for chiral reagents for use in synthesis, optical resolution, etc. In particular, the enantioselective addition of a hydroxyl equivalent (more precisely, a water equivalent) to a vinyl epoxide would generate the vinylglycidol in enantiopure form. The latter chemistry would provide a facile route to two commercially important chiral pool molecules and, as such, would be competitive with the Jacobsen procedure (i.e. kinetic resolution of the corresponding epoxides; Jacobsen, 1997a) for generation of these materials.